The present invention relates to the detection and concentration measurement of combustible gases including, for example, the detection and measurement of concentrations of combustible gases in ambient air and, more particularly, to a combustible-gas sensing method employing a gas sensing apparatus having a relatively pure noble metal or noble metal alloy sensing element that, when maintained at a predetermined elevated temperature, exhibits the ability to react with combustible gases that otherwise would have little or no affinity for the sensing element.
Current sensor methods for monitoring and controlling combustible gases and their by-products employ sensors that suffer from at least one of the following drawbacks: poor long-term stability; need for frequent maintenance and calibration; low sensitivity to the gas being monitored; and high sensitivity to common interference gases including water vapor, with the latter being perhaps the most serious deficiency for many environmental applications.
For example, semiconductor gas sensors are normally plagued by high sensitivity to interference gases present in ambient air, such as common solvents and water vapor. Especially serious is the interference caused by water vapor, which is present in virtually all ambient air and which has a similar effect on semiconductor sensors as the target combustible gases such devices are intended to detect. U.S. Pat. No. 4,911,892 to Grace et al. (the '892 patent) discloses a semiconductor gas detection apparatus having enhanced capability for rejecting interference gases. Although improvements to semiconductor sensors such as those disclosed in the '892 patent may help to limit error, the semiconductor gas sensor's inherent sensitivity to common interference gases remains.
U.S. Pat. No. 3,714,562 to McNerney and U.S. Pat. No. 5,010,021 to Bell et al. disclose a gas detection method using a sensor comprising a relatively pure thin noble metal film sensor element selected for its chemical affinity for the target gas to be detected (hereinafter "metal affinity" gas sensors). When a sample mixture containing the target gas is passed over the sensor, the target gas adsorbs onto the surface of the sensor element causing the sheet resistance of the element to change. This resistive change is measured and, by comparison to a data base, converted into concentration units.
Metal affinity gas sensors do not exhibit the same sensitivity to water vapor as do semiconductor gas sensors. They are, however, limited to detecting target gases that have a natural affinity for the sensor surface at ambient temperature and they cannot be used by themselves to sense dynamic changes in concentration of the target gas because the sensor element surface reacts with the target gas to form stable surface species. These stable species saturate the sensor surface and, once its surface is saturated, the sensor cannot be used until it is regenerated, for example, by heating to desorb the compounds, or by rinsing with ozone.
Combustible gases, such as most hydrocarbons, which are able to react readily with oxygen, are critical to human existence. Because they represent a major source of energy as well as raw materials, these substances are essential to many industrial activities. However, their widespread use generates various hazards ranging from pollution to toxicity and even fire and explosion. Combustion is a major source of air pollution, much of which could be reduced by timely sensing of combustion products to indicate an out-of-specification combustion condition, and, of course, explosion caused by undetected leakage of combustible gases is an all-too-common occurrence. Unfortunately, shortcomings inherent in current gas sensor methods and devices have prevented widespread use of sensors capable of continuous operation to monitor combustible gases to avoid these and other similar occurrences.
For the foregoing reasons, there is a critical need for a combustible gas sensor that can be used continuously to detect and determine the concentration of a wide variety of combustible gases in the atmosphere or in a gas stream without being subject to false alarms caused by water vapor or other interference gases, yet having high sensitivity, fast dynamic response to changes in concentration, and the ability to distinguish between a variety of combustible gases.